It is known, for example as disclosed in U.S. Pat. No. 3,280,891, to provide vertical blind apparatus having a plurality of panel support carriages movable along a track with a lazy-tong type linkage for interconnecting the carriages to control spacing between the panel support carriages as the vertical blind apparatus is traversed along the track from an open to a closed position. However, because of the necessary working clearances between the numerous levers and the pivots that interconnect the levers, the lazy-tong linkage usually exhibits a certain amount of looseness such that it does not maintain uniform spacing between the panel support carriages connected to the lazy-tong linkage. This problem is particularly aggravated in installations in which the carriage guide track is mounted at an angle to the horizontal. In such installations, the panel support carriages tend to bunch toward the lower end of the track when the carriages are traversed to an extended position across the window opening.